Runtime Context Skill | Agent Skills

Runtime Context

Agents run in different environments with different capabilities. This skill detects the current runtime so agents can use the right tools and fail clearly when a required capability is missing.

The Three Runtimes

| Runtime | How to detect | Tools available | Skills via | |---------|--------------|-----------------|-----------| | Claude Code | CLAUDE_CODE=1 env var, or Skill()/Agent()/Read tools exist | Full suite: Read, Write, Edit, Bash, Grep, Glob, Agent, Skill, WebFetch, WebSearch | Skill() tool | | Vercel Sandbox | /vercel/sandbox/ paths exist, VERCEL_SANDBOX_ID env var set | bash-tool trio: bash, readFile, writeFile, plus skill if configured | createSkillTool() from bash-tool | | Local dev | Neither of the above, process.cwd() is in user home or project dir | Whatever the app provides (usually just HTTP + AI SDK) | Direct file reads or none |

Detection Script

Run scripts/detect.sh to get a JSON summary of the current environment:

bash skills/runtime-context/scripts/detect.sh

Returns:

{
  "runtime": "sandbox",
  "has_bash": true,
  "has_skill_tool": true,
  "sandbox_id": "sbx_abc123",
  "working_dir": "/vercel/sandbox/bot",
  "node_version": "v22.x.x",
  "bun_version": "1.x.x"
}

For SOUL.md / System Prompt Authors

When writing a system prompt that works across runtimes, structure it like this:

## Environment Awareness

Before taking action, check your environment:
1. If you have `Skill()` and `Agent()` tools, you are in Claude Code
   - Use skills via `Skill(name)`, delegate via `Agent()`
   - Full file access via Read/Write/Edit
2. If you have `bash`, `readFile`, `writeFile` tools, you are in a Sandbox
   - Run scripts via `bash`, read context via `readFile`
   - Skills available via the `skill` tool if configured
3. If you have no tools, you are text-only
   - Answer from your training and system prompt context
   - Tell the user what you would do if you had tools

Do not guess which tools exist. If a tool call fails, state what tool
was missing and what capability it would have provided.

Adapting Behavior

The goal is NOT fallbacks. The goal is clarity.

Correct: "I'm in a Sandbox. I have bash and file tools but no Skill() tool. I'll read the skill instructions directly from ./skills/clawnet/SKILL.md."

Incorrect: Silently trying Skill() first, catching the error, then trying readFile, then trying fetch, then giving up without telling the user.

Capability Declaration Pattern

At the start of a conversation or task, state what tools are available:

I'm running in a Vercel Sandbox with bash, file, and skill tools.
I can: run scripts, read/write files, look up skill instructions.
I cannot: spawn subagents, use Claude Code tools, access the host filesystem.

This sets expectations immediately so users and orchestrators know what to ask for.

Reference Files

README.md — Three runtimes explained, capability matrix, bash-tool integration guide for SDK developers, skill declaration across runtimes

Agent Skills: Runtime Context

Install this agent skill to your local